Significance and Innovation: Real-time electronic monitoring is the optimal approach for understanding patterns of adherence, which are critical for determining if adherence to daily oral pre-exposure prophylaxis (PrEP) aligns with an individual's risk for HIV acquisition. Compared with earlier adherence measurement devices, next generation Wisepill technology offers significant innovations in data storage and transmission reliability, cost, and usability and is ready for implementation in resource-limited settings. Coupling Wisepill monitoring to SMS adherence feedback has great potential to support adherence at the precise time it is needed. Real-time strategies may be particularly important for young women- a population in need of self-controlled HIV prevention strategies, but one that has previously struggled with PrEP adherence. Triangulating real-time SMS measurement of sexual behavior with adherence may explain how PrEP can be used most effectively for HIV prevention (i.e., prevention-effective adherence). Approach: We propose a forward-thinking project that brings together key technologies to conduct optimized real-time adherence monitoring, adherence feedback, and measurement of sexual behavior to maximize PrEP adherence. We will establish a cohort of 314 young HIV-uninfected women initiating PrEP in Kenya, following them over two years: Aim 1. Use the next generation Wisepill device to a) measure real-time adherence and b) determine the impact of real-time SMS feedback on PrEP adherence. We will monitor adherence among all women with Wisepill. We will randomize half to receive weekly SMS feedback on Wisepill openings. The primary outcome will be execution and persistence of adherence, comparing the randomized groups. Aim 2. Determine the technical function, acceptability, cost, and validity of the next generation Wisepill device couple to SMS feedback among young African women. We will measure data transmission reliability, battery life, and other technical features; assess acceptability and usability; and estimate costs of use. Adherence validity will be determined by comparing Wisepill data with dried blood spot tenofovir levels. Aim 3. Triangulate Wisepill adherence data with real-time assessment of HIV risk behaviors and perceptions, to measure prevention-effective adherence. Through weekly SMS surveys about recent sexual behavior, use of alternate HIV prevention tools, and subjective HIV risk, we will explore the overlap of perceptions, behaviors, and PrEP adherence, which is critical for effective HIV prevention with PrEP. Investigators: This project brings together an established, internationally-recognized team of PrEP leaders with expertise in real-time adherence monitoring and intervention (Jessica Haberer), HIV prevention and epidemiology (Jared Baeten), women's health and HIV prevention in Kenya (Nelly Mugo and Elizabeth Bukusi), behavioral and social science (Kenneth Ngure), pharmacy science (Stella Nujugana), behavioral economics and risk behavior (Harsha Thirumurthy), and HIV modelling (Ruanne Barnabas).